Fix LAZY_TEST_SUITE handling and $(TEST_SUITE_LOG) recreation.

Fix LAZY_TEST_SUITE handling and $(TEST_SUITE_LOG) recreation.

* lib/am/check.am (check-TESTS): Expand `$(TEST_LOGS)' only once
in the rule command, for systems with low command line limits.
Remove $(TEST_SUITE_LOG) even in LAZY_TEST_SUITE mode.
($(TEST_SUITE_LOG)): Always recreate $(TEST_SUITE_LOG).  Mention
lazy mode in the summary output.
* tests/parallel-tests.test: Test LAZY_TEST_SUITE semantics.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>